1. Field of the Invention
The present invention relates to a bicycle and more particularly to such a seat tube having strong structure with light weight.
2. Description of the Related Art
A conventional seat tube for a bicycle is made of metal. Recently, the bicycle is developed towards reducing its weight and adopts fiber reinforced material. As shown in FIG. 1, the seat tube 1 is made of carbon fiber reinforced resin. The seat tube 1 has an upright tube 4 and a head 5 which are manufactured by winding multiple layers of pre-pregs into an elongated tube having a closed end and an open end, placing the elongated tube into a die, inflating the elongated tube through the open end to make the elongated tube abut on an inner wall of the die, and curing the elongated tube.
The inner wall of the seat tube 1 bears the air pressure during curing such that the shape of the inner wall is often irregular and the thickness of the inner wall is uneven, as shown in FIGS. 2-3. Therefore, the structure strength of the seat tube 1 is uneven and the structure failure is often generated at the weakness of the structure due to stress concentration. Besides, the outer wall of the seat tube 1 is restricted by the die, which has an inevitably manufacturing error, so that the shape of the outer wall of the upright tube 4 can not achieve a true circle. The upright tube 4 needs to be further trimmed to make the outer wall approach a true circle for conforming to the product standards. The trimming destroys the texture of the fibers and reduces the structure strength of the upright tube 4. If the quantity of the pre-pregs used in the seat tube 1 is increase for enhancing the structure strength, the weight of the product is unintentionally increased. There is always a problem as how to reduce the weight of the seat tube and increase the structure strength at the same time.